Plasma cells and plasmacytoid features in thyroid lesions

Most thyroid lesions are of epithelial origin characterized by a papillary and/or follicular pattern. However, lesions involving plasma cells or plasmacytoid cytomorphology that mimic plasma cells can rarely be encountered in the thyroid gland. Thyroid lesions involving true plasma cells include benign entities such as chronic lymphocytic thyroiditis and neoplasms including plasmacytoma. Thyroid lesions that present with plasmacytoid features include medullary thyroid carcinoma. Rendering the correct diagnosis may be difficult due to the rarity of these lesions, overlapping cytomorphologic features, and challenges selecting the appropriate ancillary studies. This review discusses those thyroid entities showing plasmacytoid cell features and emphasizes their relevance to the recent Bethesda system for reporting thyroid cytopathology and WHO classification of endocrine tumors.